The structure and molecular mechanism of pore formation by Macrophage Expressed Gene-1

Macrophage Expressed Gene-1 (MPEG-1; also termed Perforin-2) is an endosomal / phagolysosomal perforin-like protein that is conserved across the metazoan kingdom and that functions within the phagolysosome to damage engulfed microbes. Like the Membrane Attack Complex and perforin, MPEG-1 has been postulated to form pores in target membranes, however, its mode of action remains to be established. We used single particle cryo-Electron Microscopy to determine the 2.4 Å structure of a hexadecameric assembly of MPEG-1 that displays the expected features of a soluble pre-pore complex. We further discovered that the MPEG-1 pre-pore-like assemblies can be induced to perforate membranes through mild acidification, such as would occur within maturing phagolysosomes.

We next solved the 3.6 Å cryo-EM structure of MPEG-1 in complex with liposomes. Remarkably these data revealed that a C-terminal Multi-vesicular body of 12 kDa (MVB12)-associated β-prism (MABP) domain interacts with target membranes in a mode that positions the pore forming machinery of MPEG-1 to point away from the bound membrane. This unexpected mechanism of membrane interaction raises the intriguing possibility that MPEG-1 may be able to remain bound to the phagolysosome membrane while simultaneously forming pores in engulfed bacterial targets.

Professor James Whisstock

Professor Whisstock studied for his degree and PhD at Cambridge University. He moved to Monash in 1997. Currently, James is an NHMRC Senior Principal Research Fellow and Director of the ARC Centre of Excellence in Advanced Molecular Imaging.

His research interests include structural biology and drug development around immune-related complexes; these include proteases and protease inhibitors and pore forming proteins such as perforin. From a molecular perceptive his focus includes understanding embryonic patterning in Drosophila and how the perforin-like protein Torsolike controls activation of the receptor tyrosine kinase Torso.

Seminar host: Professor Richard Lewis (r.lewis@imb.uq.edu.au)